[cfi] Do not emit bit set entry for available_externally vtables.

[eugenis]

Summary

In the Itanium ABI, vtable may be emitted speculatively as an
available_externally global. Such vtable may not be present at the
link time and should not have a corresponding CFI bit set entry.

[pcc]

LGTM

[eugenis]

r258596

lib/CodeGen/ItaniumCXXABI.cpp

void ItaniumCXXABI::emitVTableDefinitions(CodeGenVTables &CGVT,
   const DeclContext *DC = RD->getDeclContext();
   if (RD->getIdentifier() &&
       RD->getIdentifier()->isStr("__fundamental_type_info") &&
       isa<NamespaceDecl>(DC) && cast<NamespaceDecl>(DC)->getIdentifier() &&
       cast<NamespaceDecl>(DC)->getIdentifier()->isStr("__cxxabiv1") &&
       DC->getParent()->isTranslationUnit())
     EmitFundamentalRTTIDescriptors();
 
+  if (!VTable->isDeclarationForLinker())
     CGM.EmitVTableBitSetEntries(VTable, VTLayout);
 }
 
 bool ItaniumCXXABI::isVirtualOffsetNeededForVTableField(
     CodeGenFunction &CGF, CodeGenFunction::VPtr Vptr) {
   if (Vptr.NearestVBase == nullptr)
     return false;
   return NeedsVTTParameter(CGF.CurGD);
 }

test/CodeGenCXX/cfi-speculative-vtable.cpp

+// Test that we don't emit a bit set entry for a speculative (available_externally) vtable.
+// This does not happen in the Microsoft ABI.
+// RUN: %clang_cc1 -triple x86_64-unknown-linux -O1 -fsanitize=cfi-vcall -fsanitize-trap=cfi-vcall -emit-llvm -o - %s | FileCheck  %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux -O1 -fsanitize=cfi-vcall -fsanitize-trap=cfi-vcall -fsanitize-cfi-cross-dso -emit-llvm -o - %s | FileCheck  %s
+
+class A {
+ public:
+  virtual ~A();
+};
+
+A a;
+
+// CHECK: @_ZTV1A ={{.*}} available_externally
+// CHECK-NOT: !{{.*}} = !{!{{.*}}, [4 x i8*]* @_ZTV1A, i64 16}